How to divide by a total from an alias column or sub query

Current working Query:

use pubs
select SUM(ytd_sales) AS YTD_Sales,
       SUM(ytd_sales) AS "Average Sales",
       titles.type AS "Type", 
       COUNT(*) AS "Books Sold"
  from titles
 GROUP BY type;

Creates 4 columns : YTD_Sales , Average Sales, Type , Books Sold

To calculate the "Average Sales" I need to take the ytd_sales / "ALIAS" Books Sold

Now I know I cannot divide by an Alias ("books sold"). Therefore, I created a sub-query

use pubs
select SUM(ytd_sales) AS YTD_Sales,
       SUM(ytd_sales/(SELECT titles.type, COUNT(*) from titles GROUP BY type)) AS "Average Sales",
       titles.type AS "Type", 
       COUNT(*) AS "Books Sold"
  from titles
 GROUP BY type;

But this gives me an error: "Cannot perform an aggregate function on an expression containing an aggregate or a sub-query."

I know there is an easier way but I am just not seeing it.